RPR False Positive: Could a Reactive Syphilis Screen Be Wrong?

Yes. An RPR false positive means the screening test is reactive even though there is no syphilis infection. The RPR is a non-treponemal test, so it detects antibodies that are not specific to syphilis, and CDC lists causes such as pregnancy, autoimmune conditions and other infections. A treponemal test is used to confirm or rule out syphilis.

This guide is for people reviewing a reactive or positive RPR result. It explains how the tests fit together and what to ask. It cannot tell you whether your own result is a false positive; only testing and a clinician's assessment can do that.

Do not wait for a routine appointment or a repeat blood test if you have new symptoms that could involve the brain, eyes or ears. CDC notes that syphilis can affect these areas at any stage. Seek same-day medical care if you notice:

• Eye pain or redness, or changes in your vision.
• New hearing loss, ringing or other noise in the ears, or dizziness.
• A severe headache, muscle weakness or trouble with movement.
• New confusion, trouble focusing or a change in personality noticed by others.
• If you are pregnant and have a reactive result, contact your maternity team promptly rather than waiting for a routine visit.

Syphilis blood testing uses two kinds of test. Non-treponemal tests, such as the RPR and VDRL, look for antibodies the body can make in response to syphilis, but also in response to some autoimmune diseases and other infections. Treponemal tests look for antibodies made specifically against the bacteria that cause syphilis. Common names on reports include TP-PA, TPHA, FTA-ABS, EIA and CIA.

CDC guidance states that using only one type of test is insufficient for diagnosis. The two types also behave differently over time. RPR results are usually reported as a titer, such as 1:2 or 1:32, and the titer is used to follow response to treatment. Most people who have had syphilis keep a reactive treponemal test for life, even after successful treatment, so a treponemal test does not show whether an infection is current or past.

A reactive non-treponemal test with no syphilis is sometimes called a biological false positive. CDC lists these possible causes:

• Pregnancy.
• Autoimmune conditions.
• Other infections, such as HIV.
• Vaccinations.
• Injection drug use.
• Older age.

Not on its own. The titer shows how far the sample could be diluted and still react, so 1:32 is a stronger reaction than 1:2. A low titer can occur with a false positive, but it can also occur with syphilis, including infection that was treated in the past. The treponemal result and your history are what separate these possibilities.

Titers are most useful when compared over time. CDC advises that repeat tests use the same method, ideally at the same laboratory, because RPR and VDRL titers cannot be compared directly. A change of fourfold, which equals two dilution steps such as 1:32 to 1:8, is considered meaningful.

Laboratories use different testing orders. In the traditional order, a reactive RPR or VDRL is followed by a treponemal test. If the treponemal test is reactive, syphilis is likely, now or in the past. If it is non-reactive, the RPR result is more likely to be a false positive, and the clinician decides whether any repeat testing is needed.

Some laboratories use a reverse order: a treponemal test first, then an RPR if it is reactive. If the RPR is non-reactive, CDC advises a second, different treponemal test. A reactive second test in someone never treated usually leads to treatment. A non-reactive second test in someone at low risk usually needs no further evaluation.

Your history matters too. Tell the clinician about any earlier syphilis diagnosis and treatment, recent possible exposures, pregnancy and known autoimmune or other conditions. Previous results are especially useful, because an old reactive treponemal test may simply reflect a treated past infection.

Syphilis screening is offered in pregnancy because untreated infection can pass to the baby. CDC recommends testing at the first prenatal visit, with repeat testing at 28 weeks and at delivery for people at higher risk or in areas where syphilis is more common. The NHS offers screening during pregnancy.

Pregnancy itself can cause a false-positive RPR. When results disagree, CDC advises a second treponemal test, ideally on the same sample. If it is non-reactive and risk is low, a false positive is more likely and repeat testing within about four weeks can be considered. Because the stakes are high, the maternity team may recommend treatment if follow-up is uncertain. Ask your team how they will decide and when you will hear.

If syphilis is confirmed, the NHS advises that current and recent partners are tested and treated, and that sex is avoided until two weeks after both you and your partner have finished treatment. Sexual health services can help notify partners without naming you. If the result turns out to be a false positive, ask the clinician whether any partner action is still needed.

A reactive screen can feel stressful and personal. It is a laboratory finding, not a judgement, and a reactive RPR can have explanations other than infection.

Bring a copy of the report and any earlier results. These questions can help you understand the plan:

• Which test was reactive: the RPR, a treponemal test, or both? What was the RPR titer?
• Has a treponemal test been done on this sample, and what did it show?
• Could pregnancy, an autoimmune condition, another infection or a recent vaccination explain the result?
• Do I need a repeat test, and when? Will it use the same method and laboratory so results can be compared?
• Do I need treatment now, and what follow-up blood tests will check the response?
• Is any advice needed for my partner or partners, and how can the clinic help with that?

On SameCase, we let patients and verified doctors share de-identified cases. Reading how others with a reactive RPR and a different treponemal result described their test sequence can help you notice what to ask, such as the titer or which confirmatory test was used. Other people's cases cannot tell you what your result means, and they do not replace testing or care from a sexual health or maternity service.